The present invention relates to the making of shrink objects such as shrink hoses, shrink sleeves, shrink caps, or the like.
Austrian Pat. No. 188,510, dated Jan. 25, 1957, discloses a method for making shrink hoses by extruding or injection-molding a hose from a thermoplastic material, but having a smaller diameter than the hose to be made. The extruded hose is then expanded by means of air pressure and cooled to fix its expanded dimension. The hose is later shrunk through application of heat and will contract to a reduced diameter for a shrink fit. The particular thermoplastic hose, e.g. a polyvinylchloride hose, does not achieve a sufficient stabile temperature and does not have an adequately elastic memory, in that it will not shrink exactly to its original dimensions established upon extrusion.
A particular product is known and traded under the designation "THERMOFIT" which is a high-density polyolefin to be used for injection-molding particular shapes. These parts are subsequently subjected to high-energy electron rays in order to obtain a cross-linked, three-dimensional lattice assembly of the molecules. Such a shape is quite strong mechanically, is creep-resisting, does not tear, and has an elastic memory. Upon making a hose or sleeve in that fashion and slipping it upon the object to be covered, shrinkage is obtained by briefly heating the sleeve above the crystallization point, at about 135.degree. Celsius. The sleeve will, thereupon, return rapidly to its original shape and dimension, and a truly strong cover is obtained.
The method as described in the preceding paragraph is applicable to other base polymers, also to modified polymers, depending upon any special requirements during its use. The critical aspect of this method, however, is the cross-linking by means of electronic beams, prior to heating and expansion. Such a method is, therefore, quite expensive for reasons of the requisite equipment, and also for reasons of providing adequate protection for persons involved.